In recent years, consumer demand and legislation requirements have promoted diesel engine technology advances resulting in improvements in energy efficiency and performance; and reductions in emission levels. These advances have largely been consequent of combustion process improvements achieved through finely divided atomisation of the fuel prior to combustion. This atomisation is typically achieved through the use of high pressure fuel injection systems and highly sophisticated electronic injectors—usually with an increase in the number; and a reduction in the size of the injector holes over those previously employed.
Critically, however, in these new injector systems, the negative impact of injector fouling or coking becomes far more significant. Fouling occurs where deposits occur in the internal passages or surfaces of the injector or could even form in other parts of the fuel delivery system. These deposits increase with degradation of the fuel and typically take the form of carbonaceous coke-like residues or sticky gum-like residues. This blocking or fouling results in less efficient fuel delivery and poor mixing with air prior to combustion. It is further exacerbated in injectors that have very small holes—where the threshold size for a deposit to have a substantial impact on performance is much reduced. Furthermore, within the injector body, there can be very small clearances between moving parts; where the impact of deposit formation can cause injectors to stick, particularly in the open position. As a result of these effects, injector fouling is known to lead to multiple problems such as power loss, increased emission levels and reduced fuel economy.
As previously discussed, high pressure fuel injection systems are also core to the recent performance improvements associated with this type of engine. In common rail systems, for example, the fuel is stored at high pressure in the central accumulator rail prior to being delivered to the injectors. Any unused heated fuel is then returned to the fuel tank, where it will then be introduced back into the accumulator rail on demand. Fuel being returned to the fuel tank via this route has been measured to have a temperature in excess of 100° C.
At the injector nozzle, the fuel pressure is commonly in excess of 1000 bar; and may be in excess of 2000 bar. Furthermore, as the fuel is circulated through the injector body itself, it is heated further due to heat conducted through the injector body from the combustion chamber. The temperature of the fuel at the tip of the injector can be as high as 250-350° C.
The high pressures inside these fuel delivery systems can also lead to a further source of stress on the fuel. Cavitation bubbles can form in the fuel because of the very low static pressure that occurs in high speed nozzle flow near a sharp inlet corner. The sharper the corner and the higher the velocity, the more likely cavitation is to occur. The formation of cavitation bubbles in common rail diesel injectors is well-documented. Typically, this has focused on the potential for mechanical damage or impact on injector performance; however, the implosion of cavitation bubbles must also have an impact on the stability of the fuel due to the extraordinarily high pressures and temperatures generated during this event.
Hence the diesel fuel in a common rail diesel engine is stressed:                at pressures of over 1000 bar; and        at temperatures of up to 100° C. prior to the injection eventand can be recirculated back within the fuel system thus increasing the time for which the fuel is exposed to these conditions. It can further experience cavitation during passage through the injector nozzle, which can potentially initiate instabilities in the fuel.        
Diesel fuels become more unstable the more they are heated, particularly if they are heated under pressure. Thus diesel engines having high pressure fuel injection systems will typically exhibit increased fuel degradation and hence increased injector fouling over that observed in older technology engines.
Whilst injector fouling as a result of these factors may occur with any type of diesel fuel, some fuels can be particularly prone to this problem. For example, fuels containing biodiesel have been found to exhibit increased injector fouling. Diesel fuels containing metallic species may also experience increased deposit formation. Metallic species may be deliberately added to a fuel in additive compositions or may be present as contaminant species. Transition metals in particular cause increased deposits, especially copper and zinc species.
Modern diesel engines which incorporate a high pressure fuel injection system and typically also more sophisticated injector nozzle designs are therefore both more sensitive to injector fouling problems than those utilising older diesel technology; and more likely to experience significant injector fouling in the first place.
Typically these issues are addressed through the use of specialised detergency additives in the fuel composition. For example, PCT patent application WO2009/040586 discloses the use of at least 120 ppm of a nitrogen-containing detergency additive in a diesel fuel in order to improve the performance of a high pressure fuel system in a diesel engine by reducing injector fouling. However, the use of additives has a cost implication for fuel formulation and may also have concomitant detrimental effects on other aspects of fuel performance or behaviour.
PCT patent application WO2003/091364 discloses the use of Fischer-Tropsch derived distillate or gas oil fuel in a diesel blend in order to reduce engine fouling due to combustion-related deposits. This application discloses a fouling-related behaviour benefit for incorporating FT-derived distillate in the fuel with a focus on combustion-related fuel effects. Engine fouling (even specifically injector fouling) in indirectly injected engines is typically observed to be related to the combustion properties of the fuel. An analysis of the experimental data provided in this application indicates that in order to reduce the relative fouling behaviour of the fuel blend to 50% (i.e. midway between the fouling behaviours of the crude-derived and FT-derived blend components) an amount of FT-derived diesel significantly in excess of 60% by volume (ca. 70 volume %) is required. Such a blend is expected to have a density significantly less than 0.790 g.cm−3, rendering it less useful as a commercial fuel (where typical commercial specifications require minimum densities of 0.80 g.cm−3 (at 15° C.) or even 0.81 g.cm−3 (at 15° C.)).
The inventors have determined, however, that in the case of high pressure directly injected diesel engines, moderate amounts of a highly paraffinic distillate fuel can surprisingly be used to provide significantly improved performance in terms of reducing injector fouling, whilst still providing a blend that is commercially useful by virtue of its higher density.